1. Field of the Invention
The present invention relates to an apparatus and a method for introducing secondary air heated in a heater into an exhaust system of an internal combustion engine so as to clean exhaust emission or gases emitted from the internal combustion engine.
2. Description of the Related Art
In general, the exhaust system of an internal combustion engine mounted on a vehicle has a cleaning catalyst such as a ternary catalyst for cleaning the exhaust gases. When the engine is started up from cold state, secondary air is introduced into the exhaust system so as to improve the efficiency of cleaning of the exhaust gases by the catalyst.
The cleaning efficiency of the cleaning catalyst is low when the engine is started up, because the temperature of the exhaust gases, as well as the temperature of the cleaning catalyst, is still low. The present applicant therefore has proposed a secondary air introduction system which heats the secondary air to be introduced into the exhaust system so as to further improve the cleaning efficiency during starting up of the engine.
FIG. 12 is a diagram showing various systems around an internal combustion engine equipped with the above-mentioned secondary air introduction system. An internal combustion engine 1 has an intake system including an intake pipe 2 and an exhaust system including an exhaust pipe 3. The engine further has a throttle valve 5, a catalyst case 6 provided in the exhaust pipe 3 and containing a cleaning catalyst for cleaning the exhaust gases, a transmission 7, and a secondary air introduction apparatus 8 for improving efficiency of cleaning of the exhaust gas by the cleaning catalyst particularly when the engine is being started up. The secondary air introduction apparatus 8 is adapted to supply secondary air to a portion of the exhaust pipe 3 upstream of the catalyst case 6 at a predetermined rate.
The secondary air introduction apparatus 8 will be described in more detail. The secondary air introduction apparatus has a secondary air introduction pipe 10 which is connected at its one end to the exhaust pipe 3, an air pump 11 which supplies secondary air to the other end of the secondary air introduction pipe 10, a control valve disposed in the secondary air introduction pipe 10 and adapted for controlling the rate of supply of the secondary air through the secondary air introduction pipe 10, a check valve 13 disposed in the secondary air introduction pipe 10, a heater 14 which is disposed at a portion of the secondary air introduction pipe 10 near the junction between the pipe 10 and the exhaust pipe 3, and a controller 15 for controlling the operations of the air pump 11, control valve 12 and the heater 14.
Numeral 16 designates a battery for supplying the air pump 11 and the heater 14 with electrical power, while 17 designates an air pump relay for starting and stopping the air pump 11 by connecting and disconnecting the power supply line between the battery 16 and the air pump 11 in accordance with instructions given by the controller 15. Numeral 18 designates a heater relay for starting and stopping the operation of the heater 14 by connecting and disconnecting the power supply line between the battery 16 and the air pump 11 in accordance with instructions given by the controller 15.
A description will now be given of the operation of the secondary air introduction apparatus 8 with specific reference to FIG. 13 which illustrates the manner in which the rate of supply of the secondary air by the air pump 11 varies in relation to time.
The controller 15 operates to turn the heater relay 18 on simultaneously with the start of the internal combustion engine 1. As a result, electrical power is supplied from the battery 16 to the heater 14 thereby activating the heater 14. The controller 15 also turns the air pump relay 17 on simultaneously with or with a certain time lag after the start of the internal combustion engine 1. Consequently, electrical power is supplied from the battery 16 to the air pump 11 to start the air pump 11. The controller 15 also operates to open the control valve 12 to a predetermined degree of opening simultaneously with the start of the internal combustion engine 1. Consequently, secondary air is supplied at a predetermined rate by the air pump 11 into the secondary air introduction pipe 10 via the control valve 12 as shown in FIG. 13. The secondary air reaches the heater 14 via the check valve 13 so as to be heated by the heater 14 up to a predetermined temperature and is then introduced into the exhaust pipe 3.
The heated secondary air thus introduced into the exhaust pipe 3 flows into the catalyst case 6 together with the exhaust gases emitted from the internal combustion engine 1. The cleaning catalyst in the catalyst case 6 promotes reactions between the hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gases and oxygen contained in the secondary air, thus cleaning the exhaust gases. The heated secondary air serves to elevate the temperature of the exhaust gas which is low immediately after the start up of the internal combustion engine 1. Consequently, the temperature of the cleaning catalyst inside the catalyst case 6 is raised correspondingly to enhance the activity, thus improving efficiency of cleaning of the exhaust gases.
The operation of the secondary air introduction apparatus 8 is stopped when the temperature of the exhaust gas is raised to a level high enough to sufficiently activate the cleaning catalyst in the catalyst case 6. More specifically, the controller 15 operates to turn the air pump relay 17 and the heater relay 18 off, thus terminating the operations of the air pump 11 and the heater 14. At the same time, the controller 15 operates to fully close the control valve 12.
The secondary air introduction apparatus 8 of the type described has a drawback in that there is no means for enabling detection of any trouble such as operation failure of the air pump 11, accidental disconnection of the secondary air introduction pipe 10 from the exhaust pipe 3 or sticking of the control valve 12 in the close position. Such a trouble may inconveniently lead to an accident such as shortening of the life of the heater 14 due to overheat or to such an undesirable state that the secondary air introduction apparatus 8 is used without producing any effect of cleaning until the trouble is found.
The heater 14 is connected to chassis (not shown) of the vehicle. Frequency or mode of vibration of the vehicle chassis is different from that of the exhaust pipe 3. In order to absorb such a difference in the mode of vibration, the air introduction pipe 10a interconnecting the heater 14 and the exhaust pipe 3 is usually constituted by a metallic pipe having a flexible construction. The introduction pipe 10a, therefore, tends to be damaged in a short time due to vibratory force applied thereto as a result of the difference in the vibration mode between the vehicle chassis and the exhaust pipe 3.
Japanese Utility Model Laid-Open No. 47-21018, as well as Japanese Patent Laid-Open No. 59-60019, discloses an apparatus in which a ternary catalyst and an oxidizing catalyst are disposed in the exhaust pipe of an internal combustion engine and means are provided for supplying secondary air to the region upstream of these catalysts as shown in FIG. 14. More specifically, referring to FIG. 14, a mechanical air pump 11a is provided in the intake pipe 2 of an internal combustion engine 1. A catalyst case 6a encasing a ternary catalyst as a pre-catalyst and a catalyst case 6b encasing an oxidizing catalyst the main catalyst are connected in series in the exhaust pipe 3. A secondary air introduction pipe 10, which is connected at its one end to an air pump 11a, is branched into branch pipes which are connected to portions of the exhaust pipe 3 upstream of the catalyst cases 6a and 6b. A control valve 12a is provided at the branching portion of the air introduction pipe 10.
The operation of the air introduction apparatus for the internal combustion engine shown in FIG. 14 is as follows. The internal combustion engine 1 operates with a mixture of a fuel and air which is sucked through an air cleaner 4. The throttle valve 5 controls the rate of supply of the mixture and, hence, the output power of the internal combustion engine 1. The output power of the internal combustion engine 1 is transmitted to driving wheels of the vehicle through the transmission 7. The air/fuel ratio of the mixture is so small that the rates of emissions of carbon monoxide (CO) and the hydrocarbon (HC) are large, while the catalyst temperature has not reached a level for sufficiently activating the catalyst, immediately after the start of the engine. The air pump 11a is therefore started by the power transmitted from the engine through, for example, a driving belt, so as to supply secondary air into the portion of the exhaust pipe 3 upstream of the catalyst case 6a via the secondary air introduction passage 10. Consequently, the exhaust gas with excess air is introduced into the catalyst case 6a and the catalyst case 6b so as to make reaction with the oxygen contained in the excess air, whereby the noxious CO and HC are converted into carbon dioxide (CO2). Consequently, the exhaust gases are cleaned before relieved into the atmospheric air.
When the temperatures of the ternary catalyst and the oxidizing catalyst in the catalyst cases 6a and 6b are elevated to sufficiently activate these catalyst, the control valve 12a operates so as to introduce the secondary air only to the portion of the exhaust pipe 3 upstream of the catalyst case 6b, so that nitrogen oxides (NOx), CO and HC in the exhaust gases are removed by the ternary catalyst in the catalyst case 6a and fractions of CO and HC which could not be removed by the ternary catalyst are removed by the oxidizing catalyst in the catalyst case 6b, whereby cleaned emissions are discharged into the atmospheric air.
Thus, the secondary air introduction apparatus suppresses emission of noxious components in the exhaust gases such as carbon monoxide and hydrocarbon into the atmospheric air, particularly when the temperatures of the ternary catalyst and the oxidizing catalyst have not reached the levels necessary for activating these catalysts immediately after the start up of the internal combustion engine 1.
The described secondary air introduction apparatus suffers from the following problem. Namely, since the secondary air introduction pipe 10 is branched at the portion where the control valve 12a is disposed, the total length of the secondary air introduction passage 10 is inevitably increased, with the result that the temperature of the secondary air tends to be lowered as compared with the temperature of the exhaust gases from the internal combustion engine 1. Consequently, the temperatures of the ternary catalyst and the oxidizing catalyst are lowered to undesirably suppress the exhaust gas cleaning reaction in each catalyst.